"Do any of you expect the Republicans not to go after you if you vote against this bill?" he asked during the closed-door session, according to a Democrat in the room.
At the end of his speech, Obama got a rousing ovation for saying, "I am absolutely confident that when I sign this bill in the Rose Garden, each and every one of you will be able to look back and say, 'This was my finest moment in politics.'"
Later in the Rose Garden, Obama repeated his message to Congress publicly.
"I reminded them that opportunities like this come around maybe once in a generation. Most public servants pass through their entire careers without a chance to make as important a difference in the lives of their constituents and the life of this country. This is their moment, this is our moment. ... This is our moment to deliver," the president said.
The legislation would require most Americans to carry insurance and provide federal subsidies to those who otherwise could not afford it. Large companies would have to offer coverage to their employees. Both consumers and companies would be slapped with penalties if they defied the government's mandates.
Insurance industry practices, such as denying coverage on the basis of pre-existing medical conditions, would be banned, and insurers would no longer be able to charge higher premiums on the basis of gender or medical history. In a further slap, the industry would lose its exemption from federal antitrust restrictions on price gouging, bid rigging and market allocation.
At its core, the measure would create a federally regulated marketplace where consumers could shop for coverage. In the bill's most controversial provision, the government would sell insurance, although the Congressional Budget Office forecasts that premiums for it would be more expensive than for policies sold by private firms.
The bill is projected to expand coverage to 36 million uninsured, resulting in 96 percent of the nation's eligible population having insurance.
To pay for the expansion of coverage, the bill cuts Medicare's projected spending by more than $400 billion over a decade. It also imposes a tax surcharge of 5.4 percent on income over $500,000 in the case of individuals and $1 million for families.
Critical support for the bill came earlier in the week notably from the influential senior citizens lobbying group AARP and the doctors' lobbying group the American Medical Association.
ABC News' John Parkinson, Sara Just and The Associated Press contributed to this report.